<!DOCTYPE html>
<html lang="en">

  <head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>案例二</title>
  </head>

  <body>

  </body>
  <script>
    // 案例2
    var num = 10;

    function fn() {
      console.log(num);
      var num = 20;
      console.log(num);
    }
    fn();

    //执行过程
    var num;

    function fn() {
      var num;
      console.log(num); //undefined
      num = 20;
      console.log(num); //20
    }
    num = 10;
    fn();
  </script>

</html>